Liquid level detection devices employing a closely linked heated sensor and an unheated sensor have been available for some time. They may take on various forms, the one of primary interest in this case being short probes extending through the wall of a container or conduit and comprising resistance temperature devices (RTD's). The unheated RTD provides a signal representative of the temperature of the media, normally a fluid, in which it is immersed. The heated RTD provides a signal representative of its temperature as modified by the addition of heat and by the thermal dissipation characteristics of the gases, fluid, granulated solids, powdered materials and fluidized beds in which it is immersed.
For example, if both sensors are in air, the heated RTD will be substantially higher in temperature and provide a substantially different signal represented by its temperature than is true of the unheated RTD. If both sensors are immersed in water, the water dissipates the heat from the heated RTD to a much greater extent than does air. The differential temperature between the two RTD's immersed in water is substantially less than when the sensors are surrounded by air. Of course, the unheated RTD is not affected by the characteristics of the substance in which it is immersed since it reflects the temperature of its environment, the media in which it resides.
The general principles set forth above apply when the sensors are employed to determine the level of any substance with respect to another, that is, liquid with air above it, or a heavy material with water above it. However, when the liquid and material constitute waste materials, or if the liquid is not pure water, other factors are present which could affect the accuracy of the output of the sensors. For example, the water could contain a colloid and granulated material simultaneously. The colloid can settle on the sensors and create an undesirable but similar differential temperature as that caused by the granulated material. The unit would thereby falsely signal that granulated material was present when, in fact, only colloids are present.